1. Field of the Invention
This invention relates to a power transmission belt and its manufacturing process, and in more detail to improvements to a power transmission belt the matrix of which is made of an elastomer material such as low viscous liquefied urethane and the like, and the method of manufacture.
2. Description of the Prior Art
As conventionally used, power transmission belts of a small size and lightweight are well-known as, for example, a timing belt and a cog belt composed of elastomer material such as urethane.
Since the matrixes of these kinds of belts are made of liquefied elastomer material having a low viscosity so as to permeate through a covering fabric, it is extremely difficult to form a belt having a tooth part surface covering layer which can be derived by the manufacturing process of pressurizing the teeth part using lightly viscous elastomer material.
Namely, when forming toothed faces of a belt by injecting fabric for covering the toothed face with elastomer material in the grooves of an internal die, thus forming tooth parts of the belt under a compressive force, it is hard to forma layer for covering tooth parts since the liquefied elastomer material permeates through the fabric layer.
In addition, when the belt is formed by the abovementioned process, there occurs a problem of high costing caused by the need of non permeation-proof treatment of the covering fabric.
Therefore, a power transmission belt having a tooth part, such as a timing belt, a cog belt and the like, is formed by using liquefied elastomer material with low viscosity and is generally a raw edge belt, which does not provide a covering fabric.
However, in a power transmission belt with a raw edge, as shown in Fig.7A and Fig.7B, tensile cords (b) embedded in a belt (a) are exposed at tooth bases (c). Therefore, in regard to the timing belt (Fig.7A) there may occur wearing or breakage resulting from contact of the tensile cords with top faces (not shown) of a toothed pulley, so there is a problem in that a long usable life can not be derived.
In a cog belt (Fig.7B) trained to a V-pulley, there is also a problem in that slip easily occurs due to insufficient strength under lateral pressure when a high load is applied, and a permanent deformation may occur.
In the raw-edge belt, particularly in the timing belt where tensile cords (b) may directly contact toothed pulleys, a composition protecting the tensile cords is required.
Thus, in the timing belt as shown in FIG. 8A and FIG. 8B it is proposed and put into practical use to provide an elastomer layer (d) at the tooth bases (c) for protecting the tensile cords (b). (For example, Japanese Specification No. 58-38187)
However, in any timing belt (a) composed as shown in FIG. 8A and FIG. 8B, dimpled striae (e) remain on the elastomer layer (d) as an impression of ribs (not shown) provided on top faces of an internal die used to embed the tensile cords (b) into the elastomer layer (d) during the manufacturing process. Therefore, there is a problem in that cracks may be formed resulting from the stress concentration at these dimpled striae (e) under certain conditions.
On the other hand, a belt intermixed with high strength short fabrics in the elastomer matrix is disclosed in Japanese Sepcification No. 55-132441 as a power transmission belt which is composed to prevent the formation of cracks with reinforced tooth parts.
Nevertheless, even in the elastomer matrix intermixed with short fabrics, dimpled striae (e) would still exist on the surface of the tooth bases as shown in FIG. 8A or FIG. 8B since the ribs of the top faces are needed for the manufacturing process, still leaving the problem basically unsolved.
Moreover, the nonwoven fabric layer seems to be easily formable near the surface of the timing belt, yet it cannot be in practical use, because in that case the abovementioned non permeation-proof treatment for elastomer material is required in the nonwoven fabric layer.
In the case of the cog belt, the abovementioned composition for protecting the tensile cords (b) is not particularly required since there are not so many problems as in the timing belt. However, improvement of its strength under anti-lateral pressure is needed, especially for a small sized and lightweight transmission device.